A Novel Role of the Aryl Hydrocarbon Receptor in Hepatic Steatosis

描述（由申请人提供）：肝脂肪变性或脂肪肝与代谢综合征密切相关。了解肝脂肪变性的机制将有助于预防和治疗这一常见的医学问题。芳烃受体（AhR）在肝脏中高度表达，是一种转录因子，最初被克隆为“异种受体”。AhR通过结合靶基因启动子中的二恶英反应元件（DREs）来调节外源性酶的表达。随后的研究表明，AhR也可能通过影响生理而具有内源性功能，但其内源性功能的分子机制尚不清楚。我们的初步结果表明：(1)表达组成型激活AhR （CA-AhR）的转基因小鼠在饲料中表现出肝脏脂肪变性，这种表型在使用AhR激动剂的野生型小鼠中也有重现；(2) AhR在转基因小鼠中的激活诱导CD36/FAT的表达，CD36/FAT是一种脂肪酸转运体，在肝脏脂肪酸摄取和脂肪变性中起重要作用；(3)在AhR激动剂处理的野生型小鼠中，CD36基因表达也被激活，而在AhR-/-小鼠中，这种激活被消除；(4) AhR激动剂处理人肝癌细胞可诱导CD36的表达，增加游离脂肪酸的摄取；(5) AhR激活小鼠和人CD36基因启动子；(6)激活AhR抑制极低密度脂蛋白(VLDL)-甘油三酯分泌；(7)活化AhR抑制过氧化物酶体脂肪酸2氧化；8) CA-AhR转基因C57BL/6J第三代回交小鼠出现自发性脂肪变性和糖耐量降低的迹象。根据我们的初步数据，我们假设AhR的激活通过多种机制促进肝脂肪变性，包括激活脂肪酸转运体CD36、抑制脂肪酸氧化和抑制甘油三酯的输出。通过使用“功能获得”的CA-AhR转基因、“功能丧失”的AhR-/-和CD36-/-以及AhR配体处理的野生型小鼠，我们提出了四个具体目标：(1)确定AhR的激活是否足以和必要地诱导肝脂肪变性；(2)表征ahr诱导的肝脂肪变性；(3)确定脂肪酸转运体CD36对AhR的脂肪变性作用是否必需；(4)确定AhR调控CD36表达的分子机制。据我们所知，目前的研究首次尝试确定AhR在肝脂肪变性和相关代谢异常中的病理生理作用。四环素诱导的AhR转基因小鼠，即使维持在饲料中也表现出脂肪肝，代表了一种新的、方便的和可逆的非酒精性脂肪性肝病（NAFLD）模型。希望本研究结果有助于建立AhR及其靶向脂肪酸转运体作为治疗人类脂肪肝的新靶点。

DESCRIPTION (provided by applicant): Hepatic steatosis, or fatty liver, is strongly associated with metabolic syndrome. Understanding the mechanism of hepatic steatosis will help to prevent and treat this common medical problem. The aryl hydrocarbon receptor (AhR), highly expressed in the liver, is a transcriptional factor originally cloned as a "xenobiotic receptor." AhR regulates the expression of xenobiotic enzymes by binding to the dioxin response elements (DREs) present in target gene promoters. Subsequent studies suggest that AhR may also have endobiotic functions by affecting physiology, but the molecular mechanism for the endobiotic function of AhR remains largely unknown. Our preliminary results showed that: (1) Transgenic mice expressing the constitutively activated AhR (CA-AhR) exhibited hepatic steatosis when maintained in chow diet, a phenotype that has been recapitulated in wild type mice treated with the AhR agonist; (2) Activation of AhR in transgenic mice induced the expression of CD36/FAT, a fatty acid transporter important for hepatic fatty acid uptake and steatosis; (3) Activation of CD36 gene expression was also seen in wild type mice treated with the AhR agonist and this activation was abolished in AhR-/- mice; (4) Treatment of human hepatoma cells with AhR agonist induced the expression of CD36 and increased the uptake of free fatty acids; (5) The mouse and human CD36 gene promoters were activated by AhR; (6) Activation of AhR inhibited very-low density lipoprotein (VLDL)-triglyceride secretion; (7) Activation of AhR suppressed peroxisomal fatty acid 2-oxidation; and 8) CA-AhR transgenic mice in third generation of backcross to C57BL/6J showed spontaneous steatosis and signs of compromised glucose tolerance. Based on our preliminary data, we hypothesize that activation of AhR promotes hepatic steatosis through multiple mechanisms, including the activation of fatty acid transporter CD36, suppression of fatty acid oxidation, and inhibition of export of triglycerides. By using the "gain-of-function" CA-AhR transgenic, "loss-of-function" AhR-/- and CD36-/-, and AhR ligand-treated wild type mice, we propose four specific aims: (1) To determine whether activation of AhR is sufficient and necessary to induce hepatic steatosis; (2) To characterize AhR-induced hepatic steatosis; (3) To determine whether the fatty acid transporter CD36 is necessary for the steatotic effect of AhR; and (4) To determine the molecular mechanism by which AhR regulates the expression of CD36. To our knowledge, the current study represents the first attempt to determine the pathophysiological role of AhR in hepatic steatosis and associated metabolic abnormalities. The tetracycline inducible AhR transgenic mice, exhibiting fatty liver even when maintained in chow diet, represent a novel, convenient and reversible model of nonalcoholic fatty liver disease (NAFLD). It is hoped that results from this study may help to establish AhR and its target fatty acid transporter as novel therapeutic targets for fatty liver in human patients.